1. Field of the Invention
This invention relates to a digital signal processing system for compressing a video signal by employing an orthogonal transform, especially a discrete cosine transform (DCT), and filtering.
2. Description of the Related Art
A video signal, as compared with an audio signal, is about 1000 times wider in the bandwidth, and has a sampling frequency that may sometimes reach the megahertz range. Additionally, the video signal has a two-dimensional data structure, and its quantity of data is enormous. Accordingly, a high speed operation is required, particularly in the digital image signal processing system.
As examples, the transversal filter and DCT widely employed in image signal processing are briefly explained below.
In image signal processing, a transversal filter as expressed in formula (1) is widely employed. ##EQU1##
This is generally known as convolution. To realize this filter in real time by using only 9 taps, 9 multiplication operations and 8 addition operations are required within one sampling period of the video signal. Assuming the sampling period to be 100 ns, very fast multipliers and adders are therefore needed.
Formula (1) relates to a one-dimensional filter. In the case of a two-dimensional filter, the formula becomes as shown in equation (2) below, in which a much faster operation speed is demanded as the quantity of the data to be handled is increased. ##EQU2##
On the other hand, the one-dimensional DCT is generally expressed as formula (3) below. ##EQU3##
Supposing N=4, formula (3) may be rewritten into the following matrix formula. ##EQU4##
If, for example, the data size is 8 pixels, in order to realize the one-dimensional DCT in real time, the direct calculation method by matrix operation requires 64 multiplication operations and 56 addition operations while data of 8 pixels are being entered.
The two-dimensional DCT can be realized by applying the one-dimensional DCT repeatedly in the lateral and vertical directions, but, a much faster operation speed is also needed.
Recently, various high speed devices for image signal processing have been disclosed. In using these devices, however, it is considerably difficult to realize the digital image signal processing in real time as stated hereabove. Additionally, to enhance the speed of the digital image signal processing, an exclusive system with a pipeline structure is often used, but it is likely to be functionally limited and very expensive.